I'm going to be using ULN2803APG in a design of mine, and I just noticed on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma.
Does that mean that the voltage drop across the Vce will typically be
1.3 at that current? Or does that mean the voltage for Vbe needs to be that high to have saturation? [1]
'Fraid so. It's a Darlington, intended for high voltage outputs.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
That was what I was afraid of. I should still have enough headroom, but that is wasting a bit of power...
Any suggestion on a replacement part? I need to enable/disable 8 lines with logic level inputs.
It can even be a 1-of-8 decoder I just need it to support up to 1A, and I'd like it to be relatively cheap. My load is going to 24 parallel LEDs (3.3v Vf for 16 of them, 1.8 Vf for 8), each with a constant current sink in line with them.
Cheap, for me, is < $1.25 for 8-in-8-out, or < $2.0 for 3-in-8-out (since I'd be able to replace two chips in that case).
Thanks, Daniel.
Hello Daniel
How about using power MOSFETs?:
e.g. DMN6040SSD:
60Vmax, 4.1Amax, 55mOhm max, cheap (normally twice as expensive) (two in each so-8), Logic Level Gate (means fully on while Vgs
I should add, I'm talking about price for quantities < 20 at the moment, not including shipping.
You'd think that somebody would make a MOSFET version, but AFAIK they don't.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
e t d ,
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TI makes something they call "power logic" but is mostly shift registers
Though I'd think something like
-Lasse
That could be OK, but note the threshold voltage could be as high as 3V.
That's fine for my case. I'm using an 74HCT238 @ 5V to run the thing.
Somebody, I think TI, makes an LED driver with eight built-in current sinks and a serial interface. It was mentioned here not long ago.
We like to use the TPIC6595 for driving relays and LEDs. Serial input and eight open-drain mosfets.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Then how about these:
FDN5630 24$/100 pc 2$/8 pc:
The PICs can be bought flashed:
DIY :-) 3-to-8 decoder/multiplexer 12 I/O. PIC16F505-I/SL-ND
0,66$/pc @ 10 pcDIY 3-to-8 (or 4-to-13?) decoder/multiplexer 17 I/O (one ADC). PIC16F527-I/SO
0,66$/pc @ 10 pc/Glenn
It does not mean that Vce will be 1.3V at that current -- you could have
350mA at 50V, for a little while.It DOES mean that the lowest that the transistor will pull the voltage down when 350mA is going through it is 1.3V.
"Saturation" in a BJT means that the Vce is as low as it's going to ever get at that collector current. Basically, the collector isn't acting like a pretty good current source any more -- instead, it's acting like a pretty good short circuit.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
One more - cheaper and way bigger:
DIY 3-to-8 (or 4-to-16) decoder/multiplexer 24 I/O (one ADC, two comparators, two op-amps). PIC16F570-I/SO
0,47$/pc @ 10 pc/Glenn
"Saturation" in a Darlington is equal to a Vbe + Vce-sat of one NPN, thus the odd-looking number. ...Jim Thompson
--
| James E.Thompson | mens |
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I love to cook with wine. Sometimes I even put it in the food.
There are other chips in the same series that have pregain and a single output stage. I seem to have misplaced the number, but just threw a bunch of them away. Great chips.
-- Many thanks, Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com Please visit my GURU's LAIR web site at http://www.tinaja.com
Why use a Darlinton at all ? You need to sink that much ? If you need to sink 2 amps and have a regular transistor with hfe of 100, all you need to do is supply 20 mA to the base. Off the top of my head, 220 ohm 1/8th watt should do it from a 5 volt line.
Not quite what you asked for, but here's a pin-compatible mosfet equivalent to the ULN2003:
Regards, Allan
Maybe what I really need is just 8 Enhancment-Mode N-Channel MOSFETs?
Such as 8 of these?
This doesn't gain me any more than the 74HCT238 decoder, which I already have a bunch of.
AFAICT, this has absolutely nothing to do with my post.
I was trying to avoid 8 times transistor+resistor in my original design. I wanted high impedance input high side driver.
So I'm thinking discrete parts are the way to go now. In the past, I was uncomfortable with SMD parts, and I was trying to make this as small as possible. Since I started this project, I've gotten far more comfortable with SMD and circuit-board layout.
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